Philadelphia Butter Cake. If you've never heard of it, are you ever in for a treat.
Philadelphia Butter Cake is a rich, buttery cake with a gooey center that's served in bar form. Judging by the picture and the name, you might be tempted to think that Philadelphia Butter Cake is similar to Gooey Butter Cake, a St. Louis specialty. But you're not quite right: while they have some similar characteristics, I'd call them more "cousins" than "twins". The Philadelphia version has two distinct differences from Gooey Butter Cake: first, the topping does not include cream cheese, and second, the base contains yeast.
If light-as-air-cakes are what you crave, then don't bother with this one. Though the flavor and appearance are in some ways quite simple, it's still definitely an indulgence. Philadelphia Butter Cake is almost indescribably rich: if you can imagine Gooey Butter cake without the cream cheese, and with a base that is somewhat like a crushed, compressed danish, then you're getting the right idea. It's like an opportunity to savor the very soul of butter in a compact, sweet square of pleasure.